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"A low side superheat greater than 30 degrees indicates too little refrigerant is in the low side.
· A condenser subcooling exceeding 15 degrees indicates too much liquid refrigerant is in the high side."

Further it states an example:

"High superheat with high condenser subcooling indicates a
restriction. Too much liquid is in the high side and too little in the low side."

Would I expect a restriction based upon the info above or could it be low on refrigerant?

Originally posted by HVAC Pro I'd say undersized piston or a TXV which isn't opening properly.

Evaporator has a sticker which reads "Unit equipped with piston #84." I'm not clear how to determine undersized piston when this came factory installed.

Could additional R-22 be added in limited quantity on the high side until the suction line temp begins to drop and then let the system stabilize and take new readings? Desire is for SH and SC readings to then be within acceptable limits or at least trending towards acceptable limits.

I would pump down or recover and look inside to see if the piston or strainer is partially blocked. Then I'd pull the piston and look on the side to see if it is the proper size. (Number will be stamped on the piston.) I had a similar problem with a system last year. Turned out the piston was much too small. The sticker was right but the piston was the wrong size. I've also seen shavings stuck in the orifice partially blocking the opening, which would of course effectively reduce it's size.

There is nothing more frightful than ignorance in action....Mark Twain

No frost evident. Liquid Line just as it enters evaporator is cold to the touch but no frost. I would not expect a restriction and suspect undercharge but was hopeful Superheat and Subcooling info in the original post would provide basis for some sound diagnostics advice from this forum.

Can you get the CFM and Static pressure for the supply duct also check the return duct for proper sizing and restriction. Low airflow (blower set to incorrect speed/dragging blower/restricted supply or return duct/etc.) can make it appear to be undercharged. 4 ton should be around 1600 CFM. Once proper airflow has been verified those posted readings might actually be useful. For better info list the following information; Unit design CFM and static pressure / Supply duct actual total CFM / Supply duct actual static pressure / Return duct size and return filter size with efficiency rating / Evaporator inlet WET BULB temp / outside ambient DRY BULB temp / low side pressure / low side line temp leaving evaporator / high side pressure / high side line temp at exit on condensor / temp at bottom of compressor housing / Dry Bulb temp at inlet of evap / Dry Bulb temp at outlet of evap / amp draw of compressor / condition of the klackenbocker (note: a klackenbocker can only be properly checked between the hours of 1:00 a.m. and 3:00 a.m.)

No frost evident. Liquid Line just as it enters evaporator is cold to the touch but no frost. I would not expect a restriction and suspect undercharge but was hopeful Superheat and Subcooling info in the original post would provide basis for some sound diagnostics advice from this forum.

Any further advice is greatly appreciated.

Liquid line should not be cold, should be around room temp. Is there a filter/drier on the liquid line? If so I bet your restriction is there. As others stated pull the piston and check for debris, and right size.

Originally posted by dnt Can you get the CFM and Static pressure for the supply duct also check the return duct for proper sizing and restriction. Low airflow (blower set to incorrect speed/dragging blower/restricted supply or return duct/etc.) can make it appear to be undercharged. 4 ton should be around 1600 CFM. Once proper airflow has been verified those posted readings might actually be useful.

Interesting comments as postings in the For Your Interest Fourm that speak to system efficiency discuss SH and SC but do not directly speak to CFM and Static Pressure.

I could see where that may be important in low air flow situations but the condensor, evap and blower fan are clean. Very good airflow from what I can tell although it has not been exactly measured. System is about eight years old and has been working fine until this cooling season.

I still suspect undercharge but am seeking advice from those well versed in SH and SC diagnostics.

So add some refrigerant! What are you waiting for? Use your digital scales and measure what you put in. If you don't get the results you like then go to the truck, grab the recovery cylinder and take it back out. Don't be skeeered.

There is nothing more frightful than ignorance in action....Mark Twain